1. Field of the Invention
The present invention relates generally to a mobile communication system employing a high data rate transmission scheme, and in particular, to a method and an apparatus for determining a reverse data rate.
2. Description of the Related Art
Many studies have recently been made on high data rate transmission in a CDMA (Code Division Multiple Access) mobile communication system. A major mobile communication system employing a channel structure for high data rate transmission is the HDR (High Data Rate) system. The HDR system was proposed by the 3GPP2 (3rd Generation Partnership Project 2) organization to reinforce data communication of the IS-2000 system.
A pilot channel, a forward MAC (Medium Access Control) channel, a forward traffic channel, and a forward control channel are transmitted in time division multiplexing (TDM) on a forward link in the HDR system. The set of TDM signals is called a burst. The forward traffic channel transmits user data, and the forward control channel transmits a control message and user data. The forward MAC channel transmits reverse data rate control information, reverse power control information, and information indicating whether forward data transmission will occur or not.
Unlike the forward channels, reverse channels in the HDR system are assigned to specific ATs (Access Terminals). Reverse channels for each AT include a pilot channel, a reverse traffic channel, a reverse MAC channel, and an access channel. The reverse traffic channel transmits user data, and the reverse MAC channel includes a DRC (Data Rate Control) channel and an RRI (Reverse Rate Indicator) channel. The reverse access channel is used to transmit a message or traffic to an AN (Access Network) before connection of a traffic channel.
A data rate control scheme and related channels in a mobile communication system for data transmission such as an HDR system to which the present invention will be applied, will now be described.
To control a forward data rate, each AT transmits DRC information in predetermined slots to an AN. The AN transmits data at a controlled data rate only to an AT in a good channel condition. This HDR transmission scheme remarkably increases the volume of forward link data that can be processed. With the transmission power at a maximum, the AN transmits more data per unit time during good channel conditions, and less data per unit time during poor channel conditions, by changing the length. The AN transmits data to only one AT within the AN on a transmission channel for a predetermined time period. The DRC information provides an available forward data rate calculated based on an estimated channel state in the AT.
Unlike the forward link, the reverse link allows each AT to transmit traffic at a given data rate. Here, a maximum data rate and an overload control scheme are also given. The AN notifies the AT of a maximum available reverse data rate through use of a ReverseRateLimit (RRL) message, and overload control information through use of an RAB (Reverse Activity Bit) on a MAC channel, both of which are transmitted in a forward slot.
The RAB is used to control a reverse data rate and thus control reverse link overload. For example, if the RAB is 0, the AT can either double its reverse data rate or allow it to remain at its present rate. If the RAB is 1 on the AT transmits data at or above 19.2 kbps, the AT reduces its reverse data rate by one half transmits data at or above 19.2 kbps.
FIG. 1 is a block diagram of a forward channel transmitter in a conventional mobile communication system employing a high data rate transmission scheme.
Referring to FIG. 1, the forward channel transmitter in an AN transmits a traffic channel, a preamble, a MAC channel, and a pilot channel to an AT.
After encoded in an encoder (not shown), modulated in a modulator (not shown), and interleaved in an interleaver (not shown), the traffic channel signal is punctured and repeated according to a data rate in a symbol puncturer & block repeater 101. A demultiplexer (DEMUX) 102 demultiplexes the output of the symbol puncturer & block repeater 101. For example, the DEMUX 102 converts 16 successive bits as 16 parallel channel signals. A Walsh spreader 103 spreads each of the 16 channel signals by 16 Walsh codes and a channel gain controller 104 controls the gains of the spread signals. A Walsh chip level summer 105 sums the outputs of the channel gain controller 104 at a chip level.
The preamble is repeated in accordance with the data rate in a repeater 106. A signal mapper 107 maps 0s and 1s of the output of the repeater 106 to +1s and −1s, respectively. A Walsh spreader 108 spreads the output of the signal mapper 107 with a predetermined Walsh code. A first time division multiplexer (TDM) 109 time-division-multiplexes the traffic channel signal received from the Walsh chip level summer 105 and the preamble signal received from the Walsh spreader 108 according to a TDM control signal. Referring to the pilot channel, 0s and 1s of the pilot channel signal are mapped to +1s and −1s, respectively, in a signal mapper 191. A multiplier 192 multiplies the output of the signal mapper 191 by a predetermined Walsh code and outputs a spread pilot channel signal.
The forward MAC channel transmits an FAB (Forward Activity Bit), an RPC (Reverse Power Control), and an RAB. When the FAB is generated in every frame (e.g., 26.67 ms), a bit repeater 110 repeats the FAB 15 times (the FAB occurs 16 times) to increase a search probability. For the input of the repeated FAB symbols, a signal mapper 130 generates a signal of ±1 in a real transmission form. A multiplier 150 multiplies the output of the signal mapper 130 by a Walsh code. The Walsh code is Walsh code #1 among Walsh codes of length 32 bits.
No power control is performed on the forward link of a mobile communication system like the HDR system because it transmits signals with its maximum transmission power. However, a soft handover and a power control are required on the reverse link. Therefore, an AN transmits the RPC bit as reverse power control information. The RPC bit is generated at 600 bps. A signal mapper 131 converts the RPC bit to a signal of ±1 in a real transmission form. A Walsh channel gain controller 140 multiplies the output of the signal mapper 131 by a Walsh channel gain function. The gain function applied to an RPC bit for each AT is determined according to a DRC received from the AT. For example, if a link state is very poor, the gain controller 140 determines a gain to be 0 and transmits no power control information. A multiplier 151 multiplies the output of the Walsh channel gain controller 140 by a Walsh code of length 32 corresponding to the MAC index of a corresponding AT.
A bit repeater 120 repeats the RAB according to RABLength. The transmission rate of the RAB is 600/RABLength bps and RABLength is known by a channel assignment message. A signal mapper 132 converts the RAB to a signal of ±1 in a real transmission form and a multiplier 152 multiplies the output of the signal mapper 132 by Walsh code #2 of length 32.
A Walsh chip level summer 160 sums the FAB, RPC, and RAB signals. A signal repeater 170 repeats the sum three times (the sum occurs four times) and multiplexes the repeated sum in the second half of a forward transmission slot prior to transmission to the AT. A second TDM 180 time-division-multiplexes the traffic signal received from the first TDM 105, the pilot signal received from the multiplier 192, and the output of the repeater 170.
FIG. 2A illustrates the structure of an active slot in the conventional mobile communication system employing a high data rate transmission scheme. The active slot is transmitted only when there exists traffic or control data to be transmitted on a forward channel.
Referring to FIG. 2A, a pilot channel, a MAC channel, and traffic or control data are time-division-multiplexed in the active slot. The pilot channel is used for channel estimation in a receiver. The pilot channel data can be modulated in BPSK (Bi-Phase Shift Keying). The pilot channel includes two pilot bursts per slot and each pilot burst is disposed at the center of a half slot.
The MAC channel includes an FAB, an RPC, and an RAB. Each of the three channels is modulated in BPSK and spread with a Walsh code of length 32. Since each of the channels of the MAC channel is spread to 32 chips and occurs four times, a total of 128 chips are assigned to the MAC channel in one slot. The 128-chip MAC channel is divided into two equal 64-bit bursts. The two bursts are located in TDM before and after the second pilot burst. A traffic varies depending on channel conditions. That is, the traffic can be transmitted at a variable data rate according to the reception carrier-to-interference ratio (C/I) of a receiver and the number of slots for one can be changed according to a data rate.
FIG. 2B illustrates the structure of an idle slot in the conventional mobile communication system employing a high data rate transmission scheme. The idle slot is transmitted when there is neither traffic nor control data to be transmitted on the forward channel.
Referring to FIG. 2B, in the absence of forward traffic or forward control data to be transmitted, only a pilot channel and a MAC channel are transmitted in an idle slot (or an idle frame). Each idle skirt signal of all 0s is inserted before and after a first pilot burst. The idle skirt signal is transmitted to increase an accuracy with which a reception C/I is estimated when a multi-path component of the pilot channel arrives in the AT at a different time from that of a pilot signal from another AN.
Only the pilot channel signal is transmitted in a first half of the idle slot, and the pilot channel signal and MAC channel data before and after the pilot channel signal are transmitted in a second half slot. Because the arrival time of the multi-path component of the first pilot burst differs from that of a pilot burst from another AN, the pilot bursts are at different positions on a time axis and thus interference with the first pilot burst may be measured to be lower than in reality. On the other hand, despite the difference between the arrival times of a multi-path component of the second pilot burst and the pilot burst from the different AN on the time axis, the C/I is estimated accurately due to the MAC channel in the second half slot. Accordingly, the idle skirt signals are located before and after the first pilot burst in order to increase the accuracy of estimating the C/I of the first pilot burst. The idle skirt signals are two 64-chip bursts like the MAC channel.
Now, the FAB, RPC, and RAB of the forward MAC channel will be described in detail. The FAB indicates whether a forward traffic channel and a forward control channel are activated or not on a frame by frame basis. The same FAB is transmitted in 16 slots to an AT. An FAB in an nth frame indicates whether a forward channel in an (n+2) frame is activated or not. According to the FAB, the AT can detect an AN without data transmission while estimating a C/I to generate a DRC signal. Thus, the AT can set a DRC value accurately. If the FAB is 0, it implies that there is no data to be transmitted in the (n+2) frame and if the FAB is 1, it implies that there is data to be transmitted in the (n+2) frame.
The RPC is power control information needed for reverse power control. However, no power control is required on the forward link because the maximum transmission power is used all the time.
The RAB is identical for all ATs, for control of data rates. If an RAB from at least one AN in an active set for one frame period is 1, ATs that are transmitting at or above 19.2 kbps should reduce their reverse data rates by half. If RABs from all ANs in the active set are 0s, ATs should keep their current data rates or double them. While the RAB control is advantageous in that overload is controlled by uniform control of the data rates of unspecified ATs, it may result in unnecessary data rate reduction. The indiscriminate data rate control is not appropriate when a threshold is set for RAB application and the reverse data rate of each AT is controlled according to the threshold while maintaining the reverse link capacity in the vicinity of the threshold within a controllable range of an AN.